The visual contents, whether these be fixed or moving pictures, are in general creations that benefit from guarantees of exclusivity associated with the creator's rights. Their reproduction is in general permitted only within a strictly defined framework that allows the creators and their beneficiaries to be remunerated.
To ensure that these legal rules are complied with correctly, many systems have been developed to prevent illegal copies or to make the quality of the copies sufficiently degraded to make them unusable.
Within this context, the patent application WO 05/027529 aims to combat the copying of source pictures by means of a camera while they are being displayed, for example using a camcorder in a movie theatre. In this document, it is proposed to generate, from each source picture of the sequence to be displayed, at least two successive processed pictures, in which the colour of at least one pixel in the processed pictures is modulated temporally around the colour of the pixel in the source picture and to display these processed pictures. The pixels whose colour is modified represent an anti-piracy message. The processed pictures are displayed at a high frequency that makes the message invisible to the human eye but visible in the sequence filmed by the camcorder. Such a solution requires a modulation of the colour of the pixels at a frequency higher than the critical flicker frequency, which is of around 10/20 or 50/60 Hz depending on the flicker characteristics, and therefore applies only to projection systems having a high refresh frequency, at least of around 100 Hz. It is also possible to modulate the luminance or the brightness of the pixels instead of their colour.
In such projection systems, the modulation frequency is constant and is generally half the refresh frequency. The main problem with such systems is that once a pirate has figured out what the modulation frequency is, he can configure his camcorder shutter to filter out this frequency and bypass the anti-camcorder method.